Low-level boron detection and measurement

ABSTRACT

Methods and apparatus ( 50 ) are disclosed for accurately measuring low concentrations of boron in deionized water utilizing the chemical reaction of boric acid with a polyol by injecting very small plugs of concentrated polyol into streams of boron containing and non-boron containing water samples to produce an ionized acids product, and then measuring the conductivity difference (delta conductivity), corrected for interfering or extraneous factors which can effect conductivity, between such boron containing and non-boron containing samples using a conductivity and temperature detector ( 23 ).

[0001] The present invention relates generally to methods and apparatusfor detecting and measuring low concentrations of boron in deionizedwater based on the reaction of boric acid with well-known polyols toform well ionized complexes. The formation of such well ionizedcomplexes causes dramatic increases in the conductivities of thedeionized water, which increases have been found to be mathematicallycorrelated to the concentrations of boron in the water. The method ofthis invention particularly includes injecting very small “plugs” oraliquots of concentrated polyol into boron containing and non-boroncontaining water samples, and measuring the conductivity differentialbetween the boron containing and non-boron containing samples. Aftercorrecting for the increase in conductivity attributable to the polyolin accordance with this invention and for temperature, highly accuratemeasures of low concentrations of boron can be obtained utilizing themethod and apparatus of this invention.

BACKGROUND OF THE INVENTION

[0002] It has become increasingly important in recent years to be ableto detect and measure very low concentrations of boron in deionizedwater with a high degree of accuracy and high reproducibility ofresults. For example, in certain industrial applications, such assemiconductor manufacture, even very low levels of boron in deionizedwater used in manufacturing can significantly and adversely affect thequality and performance of the resulting products.

[0003] Large amounts of ultrapure water are required in processes tomanufacture semiconductors. Boron is one of the contaminates that mustbe removed to very low concentrations. Boron is a semiconductor p-typedopant used in manufacture of solid state electronics and functions as aprincipal charge carrier in a crystal of silicon. Accordingly, boronmust not be added inadvertently during the manufacturing process. S.Malhotra et al. reported in “Correlation of Boron Breakthbrough versusResistivity and Dissolved Silica in RO/DI System” (Ultrapure Water,May/Jun. 13, 1996.(4): p, 22-26) that boron was the first ion tobreakthrough the ion exchange resin beds when they switched tothin-film-composite (TFC) reverse osmosis membranes. The introduction ofTFC reverse osmosis (RO) membranes (to replace cellulose acetate ROmembranes) was very effective in reducing the silica passage of the ROapparatus. The reduction in boron passage was not as great, however. Thefirst ions to leak from the mixed ion exchange resin beds that followthe usual RO pretreatment are weakly ionized compounds such as silicaand boron. With TFC RO membranes, silica passage is much less than boronpassage. Boron is therefore often the first ion to breakthrough mixedion exchange resin beds in water purification systems that use RO TFCpretreatment. This is especially true if the feedwater contains highlevels of boron. Adsorption of borate ion on anion exchange resins isthe most common method to remove boron from ultrapure water. As theresin begins to become exhausted, however, borate is one of the firstions to leak through. Such borate leakage can rapidly exceed acceptableconcentration levels.

[0004] Thus, there is a need in semiconductor manufacture and many otherapplications to be able to monitor very low levels of boron in waterquickly, accurately, inexpensively, and while a deionized water streamis on-line. Obtaining reliable measurement of very low levels of boronin water also requires methods and apparatus that eliminate or at leastminimize the many possible sources of small errors in conventionalapproaches, for example, unnecessary possible contamination of samples,sensitivity of readings to flow rate variations, and small changes inaqueous conductivity caused by polyol added to enhance ionization ofboric acid. In the past, such small sources of errors have largely beendisregarded as insignificant relative to the relatively highconcentrations of boron being measured. It will be apparent, however,that as it becomes necessary to measure boron at increasingly lowerconcentrations, even very small measurement errors inherent in prior artmeasurement methods become increasingly significant and lead to largemeasurement distortions on a percentage basis.

[0005] It is generally known in the art to form conductive polyol-boroncomplexes and to utilize conductometric detection techniques. Theseprior art conductometric polyol-boron complex methods, as well as priorart colorimetric methods, however, are not sensitive enough to detectvery low concentrations of boron in purified water used in semiconductormanufacturing. Also, to measure very low levels of boron in water usingprior art methods typically requires some form of pre-concentration ofboron in the water sample to achieve sensitive enough detection. Theseadded steps introduce additional errors and complexity to themeasurement process. Some of these prior art processes for borondetection and their limitations and deficiencies are discussed furtherbelow.

[0006] One prior art approach to boron measurement is ICP-MS. ICP-MSdetection of boron with a pre-concentration step is currently the mostsensitive measurement method reported in the literature. The limits ofdetection are reported to be about 0.005 ppb as B. Withoutpre-concentration, however, the limit of detection using ICP-MS is muchhigher, about 0.050 ppb as B. The cost of ICP-MS apparatus is very high,and this fact prevents its common use on-line to measure boronconcentrations in real time.

[0007] Dionex Corporation has developed an ion chromatographic method tomeasure boron concentrations. This method uses a styrene-based resinwith polyhydroxyl functional groups attached for pre-concentration. Theresin polyhydroxyl group-boron complex formation constant is smallerthan the boron-mannitol complex formation constant. This allows theresin to collect boron from a water sample; and, after such collectionis complete, boron can be removed from the resin using 100 mM mannitoland 2 mM H₂SO₄ eluent. The concentrated boron is carried to aresin-packed ion separation column. The separation column resin has ananionic charge. This allows ion exclusion separation of theborate-mannitol anion. Conductometric detection is then used to measurethe borate-mannitol complex. The limit of boron detection with thepre-concentration column was reported as 0.050 ppb as B. This system isnot well suited for on-line measurement of boron in water, however, dueto its complexity. This system of boron analysis was reported at the1998 SPWCC by B. Newton (SPWCC, pp. 197-216,1998).

[0008] P. Cohen et al. (U.S. Pat. No. 3,468,764) described anotherdesign for a boron-in-water analysis method and apparatus. Such deviceintroduces boron-containing water and, periodically, a known boronstandard, to a bed of compressed mannitol spheres, and then measures theconductivity of the resulting boron-mannitol complex. The conductivitydifference between the water sample and the boron standard isproportional to the concentration of boron in the sample. This device isused in boron concentration ranges commonly found in the nuclear powerindustry. The detector response is reported to be linear at a firstslope over a concentration range of 800 to 3200 ppm and also linear overa concentration range of 0-800 ppm but with a second very differentslope. Boron has a high radiation cross section to neutrons and is thusused to control nuclear reactors. There are a number of differencesbetween the Cohen et al. '764 patent and the present invention.

[0009] The Cohen et al. device does not take account of the necessity tomeasure the conductivity of polyol/boron-free water to obtain accuratelow level boron measurements. This oversight introduces unacceptableerrors into measurement of very low boron concentrations.

[0010] The Cohen et al. '764 patent does not compare the conductivity ofa polyol/boron-containing sample with the conductivity of apolyol/boron-free sample, a feature that is a key aspect of the presentinvention.

[0011] The Cohen et al. device adds mannitol by flowing the sample orboron standard over a bed of mannitol spheres. By contrast, in apreferred embodiment the present invention injects a small volume of aconcentrated, even saturated, polyol solution into a micro stream of aboron-containing sample or a boron-free sample. The Cohen et al. methodof polyol addition is sensitive to sample flow rate and to the instantsurface area of the mannitol bed. If residence time is too short, themannitol concentration will change. If flows could be controlled so thatmannitol concentration approached saturation, mannitol concentrationwould then be stable. Such a modification, however, is neither describedby Cohen et al. nor compatible with the present design. The Cohen et al.design is in addition generally wasteful of mannitol.

[0012] Finally, Cohen et al. do not teach the deionization of polyolsolution to remove ions that could contaminate the conductivity of thepolyol before its addition to the water sample, thereby againintroducing inconsistencies and inaccuracies into the boronmeasurements.

[0013] Ikuo Yabe (U.S. Pat. No. 4,204,259) teaches a device that can beused on the primary cooling water in a pressurized water atomic powerplant, and, therefore, is generally concerned with measuring relativelyhigh concentrations of boron. In such patent, a mannitol solution and aboron-containing water sample (or a boron standard) are blendedtogether. The conductivity of such boron-containing water sample (orsuch boron standard) is measured in a first conductivity cell beforemannitol is added. The mannitol/boron-containing sample is passedthrough a thermal correction device and then into a second conductivitymeasurement cell. This device measures lithium (Li) with the firstconductivity cell and corrects the boron measurement made at the secondconductivity cell for the Li conductivity contribution. There are anumber of differences between the Yabe '259 patent and the presentinvention.

[0014] Yabe '259 describes an invention that continuously blendsmannitol solution with a boron-containing sample (or boron standard).This method of polyol addition is also sensitive to variations betweenthe flow rates of the sample and mannitol streams, and thereforeintroduces measurements errors.

[0015] The Yabe '259 process consumes large amounts of mannitol becauseof the continuous addition method used. By contrast, the preferredmethod of polyol addition for the present invention is injection ofsmall aliquots.

[0016] Yabe '259 does not account for the necessity of correcting foradded conductivity attributable to a polyol/boron-free sample foraccurate low level boron measurements. This will introduce unacceptableerrors in measurements at low boron concentrations.

[0017] The present invention compares the conductivity of apolyol/boron-containing sample with that of a polyol/boron-free sample.Yabe '259 does not teach this critical step for boron measurement.

[0018] Finally, Yabe '259 does not teach deionization of polyol solutionbefore its addition to the water sample to remove ions that couldcontaminate the conductivity of the polyol, thereby again introducinginconsistencies and inaccuracies into boron measurements.

[0019] A Russian journal publication entitled “Flow-InjectionDetermination of Boric Acid”; O. V. Krokhim, et al., ZhumalAnaliticheskoi Khimii, Vol. 47, No. 5, pp. 773-775 (May 1992) describesuse of a flow injection method to measure high levels of boron in water.The range of boron concentrations that can be measured linearly usingthis approach is reported to be from 10 ppm B to 16,000 ppm B.

[0020] Such method has some superficial similarities to the presentinvention in that both use a flow injection method for determiningconcentrations of boron, and both also use, conductivity of apolyol/boron-containing sample water solution as a measure of the amountof boron in solution.

[0021] There are a number of critical differences, however, between themethod taught by the Russian journal article and the present invention.Because the method described by Krokhim et al. does not measurepolyol/boron-free background, it cannot accurately measure boron at verylow levels of from 0.00 ppb to 1000 ppb (1 ppm). Furthermore, the methodof Krokhim et al. describes a generally linear response range from 10ppm B to 16,000 ppm B. By contrast, the invention of the presentapplication has a mathematically-correlated response range that startsbelow 0.05 ppb B and continues up to 1000 ppb B and higher.

[0022] The method as practiced by Krokhim et al. consumes large amountsof polyol by injecting a sample into a deionized (DI) water stream andthen mixing this resultant stream with another stream of concentratedpolyol solution. By contrast, the present invention utilizes smallinjections of concentrated polyol directly into the deionized watersample, thereby consuming only a small amount of polyol to provide boronconcentration measurements. Such improved method, using small amounts ofpolyol, makes this technique compatible with on-line and compactmeasurement equipment, something that was virtually impossible toachieve with prior art boron measurement processes. The improvedefficiency of reagent consumption with the present invention also leadsto lower operating costs and smaller instrument volumes and smaller footprints in industrial production environments.

[0023] The method taught by Krokhim et al. results in a high sensitivityto flow ratio variations of the two main streams. By contrast, thepresent invention does not have this problem because concentrated polyolsolution is injected in very small aliquots into a sample. The mixingratio is accurately fixed and therefore is independent of the sampleflow rate.

[0024] Krokhim et al. also do not teach deionizing polyol solution tominimize conductivity background variations which will otherwise lead tomeasurement inconsistencies and inaccuracies.

[0025] Furthermore, Krokhim et al. do not compare the conductivity of anaqueous solution of polyol/boron-containing sample with the conductivityof a polyol/boron-free sample, a step which is critical to the presentinvention.

[0026] Unexamined Japanese patent application specification J 10-62371(published Mar. 6, 1998) (“J 10-62371” hereinafter) for “A process andapparatus for measuring boron, ultrapure water production apparatus anda process for the operation thereof” teaches measurement of boron insemiconductor-quality ultrapure water using boron-polyol complexchemistry. Polyol solution in J 10-62371 is deionized to remove ionsthat might otherwise contaminate the polyol with conductive ions. Theprocess adds a low concentration of polyol to a sample stream andmeasures the conductivity of the resulting solution at least after suchaddition. In a first version (FIG. 1 of J 10-62371) polyol is pumped outof a storage container, through a pump, through a mixed bed ion exchangeresin and into the sample stream. The reaction product is measured witha conductivity sensor. In a second variation (FIG. 2), the conductivityof the sample stream is measured before and after injection of deionizedpolyol solution. In a third version (FIG. 3), polyol is continuouslyrecirculated through a pump, a mixed bed ion exchanger, and a polyolstorage tank. On command from an electronic control unit, a second pumpand valve are activated to remove deionized polyol from therecirculating loop and add it to the sample stream.

[0027] This approach also has some superficial similarities to thepresent invention in that the detector of J 10-62371 is intended formeasurement of very low levels of boron as needed in the semiconductorindustry, and polyol is deionized before it is added to the samplestream. There are a number of critical differences, however, between theapparatus and method taught by such application and the presentinvention.

[0028] First, although the detector of J 10-62371 is intended to measurelow levels of boron in ultrapure water, the design and operation of thedevice do not lead to sensitive enough results for practical on-lineanalysis of very low boron levels due to multiple, critical deficienciesin the design and operation of such detector.

[0029] One such deficiency is that the injection of polyol in J 10-62371is continuous and not a plug injection into a tube of small internaldiameter (as in the present invention), and is therefore subject tochanges in accuracy of boron measurements due to flow rate variations. Asecond deficiency is that J 10-62371 does not teach or suggest thecritical importance of routinely correcting conductivity readings forconductivity of a polyol/boron-free sample. A third deficiency is that J10-62371 does not teach or suggest the need for using concentratedpolyol solutions to obtain the critically necessary sensitivity. Becauseof the foregoing deficiencies, the method and apparatus of J 10-62371cannot successfully and accurately measure boron concentrations at thevery low levels of the present invention.

[0030] Thus, in J 10-62371, the conductivity of a polyol/boron-freesample is neither routinely measured nor routinely used to correct theboron response of a polyol/boron-containing sample. Instead, theapproach of J 10-62371 includes the steps of measuring the conductivityof a polyol-free sample containing boron, and then subtracting thispolyol-free conductivity from the conductivity of apolyol/boron-containing sample as conductivity correction. Suchconductivity correction may be easier to carry out, but, critically, ityields very different and less accurate results as compared with theconductivity correction procedure of the present invention. That isbecause the conductivity correction method of the present inventioncorrects for conductivity effects attributable to polyol/boron-freesample water solution, while the J 10-62371 method does not do so. Useof relatively much higher concentration polyol solutions, anotherfeature of the present invention as discussed below, leads to evengreater conductivity effects attributable to polyol/boron-free samplesthereby further increasing the need for the conductivity correctionmethod of this invention.

[0031] In the process of J 10-62371, concentrations of polyol aftermixing with boron containing samples are so low they do not exertsignificant conductivity increases. The concentration of mannitolsolution at the conductivity cell in J 10-62371 is reported as 0.0060moles mannitol/l. At such concentration, conductivity background from amannitol/boron-free sample (although not even suggested by J 10-62371)is not measurable. This fact explains why Table 1 in J 10-62371 showsthe same resistivity (18.2 megaohm cm) before and after mannitol isadded (18.2 megaohm cm). By contrast, with the present invention, theconcentration of mannitol in the mannitol/boron-free sample is 0.32moles/l. At this fifty times higher concentration of polyol, theconductivity background is 5.1 μS/cm higher than the conductivity ofultrapure water without polyol (0.055 μS/cm), and therefore must beaccounted for, which is not taught or suggested by J 10-62371. Inexchange for the higher polyol concentration of the present inventionnecessitating an additional corrective step, however, the conductivityresponse per ppb of boron of the present invention is greatly increased(by 8 times at 0.05 ppb boron to 14 times at 10 ppb boron) relative tothe response obtained using the technique of J 10-62371.

[0032] The concentration of polyol solution is so low in J 10-62371 thatit causes measurement of boron to not be very sensitive. Thus, in suchapplication, polyol is added at a very dilute concentration to conservepolyol use. FIG. 13 of the present application shows the critical effectof using low concentrations of polyol on measurement of boron, comparingdata for the device of J 10-62371 with comparable data using theapparatus and method of the present invention. It can be seen that theconductivity response curve for data resulting from the J 10-62371device/process is relatively flat, making it critically difficult todetermine very low concentrations of boron over the range 0-10 ppb. Bycontrast, the conductivity response curve for comparable data generatedusing the present invention is quite steep thereby clearlydifferentiating boron concentration differences of as little as 0.05 ppbbased on critically significant changes in the conductivity. Achievingsuch critically greater sensitivity in conductivity measurements,however, has been found to require use of much more concentrated polyolsolutions, which as discussed above is completely inconsistent with thedesign and operation of the device taught by J 10-62371.

[0033] J 10-62371 does not teach that the difference betweenconductivity of a polyol/boron-containing sample and conductivity of apolyol/boron-free sample is mathematically accurately correlated to alow-level concentration of boron in such sample over the range of verylow levels of boron. J 10-62371 also does not teach that theconcentration of the polyol solution must be relatively high (i.e.,preferably greater than 0.05 M polyol/l) in order to obtain the criticalhigh sensitivity response needed for accurate low-level boronmeasurements. J 10-62371 certainly does not teach the use of both ofthese techniques in combination.

[0034] Another distinction between J 10-62371 and the present inventionis that the former does not teach or suggest injection of microlitervolumes of high concentration polyol into a boron-containing sample.Furthermore such patent application teaches continuously injectingpolyol into a continuously flowing sample stream, but this approachrequires a large volume of polyol reagent. Additionally, the method of J10-62371 is sensitive to any changes in the ratio of flow rate of polyolinjection relative to sample flow rate. By contrast, the presentinvention is not sensitive to changes in sample flow rate because in thepreferred embodiment a small plug of polyol is inserted into the samplestream, and dilution is fixed by a combination of the laminar flow,surface tension and diffusion within the micro dimensions of theapparatus.

[0035] Still another critical difference is that the present inventionuses just one pump and one valve both to recirculate polyol solutionthrough the deionization resin bed and to insert a micro-plug of polyolinto a tube containing the sample stream and having a small insidediameter. By comparison, FIG. 3 of J 10-62371 requires two pumps toachieve the same functions. Where J 10-62371 teaches use of only onepump (as shown in FIGS. 1 and 2 thereof), it does not includerecirculation of polyol through a deionization resin bed. As shown inFIGS. 1 and 2 of said application, a pump pumps polyol from polyolstorage compartment, through the pump, through a deionization module andthen into a sample stream. As a result, the methods of FIGS. 1 and 2 ofJ 10-62371 require accurate flow measurements and controls, and use alarge amount of polyol for each boron analysis.

[0036] The consumption of polyol is much less for the present inventionthan for the J 10-62371 because in the present invention polyolmicro-pulses are injected into samples. By contrast, the Japaneseapplication uses a continuously flowing stream of dilute polyol added tocontinuously flowing sample, even when polyol addition is only madeperiodically. By contrast, the present invention adds polyol accuratelyin micro-injections of only 25 μl or 50 μl. The apparatus of J 10-62371requires large tanks to store the large volume of polyol required forsuch process. Typical flow rates of polyol and sample used in J 10-62371are 1.44 /hr (24 ml/min.) and 15 l/hr (250 ml/min.) respectively. Themannitol has a concentration of 12.5 grams/liter or 12.5/182=0.069moles/l. Therefore, the method of J 10-62371 uses 0.069 M/l×1.44l/hr.=0.01 moles (18 grams) mannitol/hr. to measure boron in the sample.By comparison, the present invention uses only 50 μl of a 1.0 molar (182g/l) mannitol solution or 5×10⁻⁵ moles (0.009 grams) mannitol/injection.Based on a typical 12 injections per hour, the present invention uses6×10⁻⁴ moles (0.1 grams) mannitol per hour, only about {fraction(1/180)}^(th) the mannitol consumption of J 10-62371.

[0037] Another recent journal article entitled “Determination of Boronby Flow Injection Analysis Using a Conductivity Detector” by S. D.Kumar, et al.; Analytical Chemistry, Vol. 71, No.13 (Jul. 1, 1999) pages2551 to 2553, teaches use of an injection valve to inject a 100 μlvolume of a boron containing sample into a stream containing 0.3 Mmannitol flowing at 1 ml/minute. The combined streams are flowed into amixing tube, and then into a 6 μl volume conductivity cell whereconductivity change is measured. The linear boron measurement range ofthis method is reported as 0-20 ppm as B, and the lower limit ofdetection is reported as 10 ppb as B. When there are interfering ions inthe sample, a pre-treatment procedure to remove them is employed. Thispre-treatment procedure, however, requires stirring a strong base anionexchange resin in the Cl⁻ form with an aliquot of the sample to convertall ionized anions to the Cl⁻ form. After filtration, such pre-treatedsolution is passed through a column containing cation exchange resin inthe Ag⁺ form to remove all chloride and ionized cations. As Kumar et al.point out, the method does not remove weak acid anions such as acetate,formate and bicarbonate quantitatively.

[0038] Here again, the apparatus and method of Kumar et al. have somesuperficial similarities to the present invention in that both methodsand apparatus use flow injection analysis to measure boron in solution,and the concentration of the mannitol solution is similar in bothdesigns. Kumar et al. inject a boron-containing sample into a flowingmannitol solution whereas the present invention, critically, injectsconcentrated mannitol into a flowing sample stream.

[0039] There are numerous critical differences between the method andapparatus taught by Kumar et al. and the present invention. The dynamicrange of the analyzer of the present invention is from 0-1000 ppb as B,whereas the device of Kumar et al. measures from 0-20 ppm as B. For theKumar et al. device, the lower limit of detection (LOD) is 10 ppb as B.while the present invention has a LOD as low as 0.05 ppb as B (200 timeslower). The response factor is also critically different between the twomethods. The Kumar et al. response is 0.5 μS/ppm B. By contrast, thepresent invention shows a response factor of 17 μS/ppm B, a 34 timesimprovement.

[0040] Still another critical deficiency is that the Kumar et al. methodof injecting a sample as a slug into a flowing 0.3 M mannitol stream isnot efficient with respect to reagent consumption. If a measurement weremade every 10 minutes, the amount of polyol consumed over 6 months wouldbe 13,100 ml of 0.3 M mannitol (720 grams mannitol) calculated as (0.5ml/measurement)×6 measurements/hr.×24 hr./day×182.5 days). In contrastto such method, the method of the present invention provides converselyand critically for the injection of polyol as a micro slug directly intoa flowing sample stream (both for boron-containing water and forboron-free water). The invention of the present application requires aninjection volume of 25 μL of polyol per measurement into the samplestream. The volume of 1 M mannitol used over the same six month periodthus would be only 655 ml (120 grams mannitol). This amount is six timesless mannitol than that required for the process of Kumar et al., andtwenty times less solution. This extremely efficient use of polyol isvery important and practical for on-line boron measurements as it makespossible the use of reagent containers of a reasonable size.

[0041] Kumar et al. do not teach deionization of concentrated mannitolsolution. This step has been found to be critically important iflow-level boron concentrations are to be measured accurately. Suchfailure by Kumar et al. probably explains their LOD of only 10 ppb as B.

[0042] The present invention uses a micro conductivity cell that is onlyone-third the volume of the Dionex conductivity cell used in the Kumaret al. device. Both cells have a cell constant of one. The smallerconductivity detector volume of the present invention improves theconductivity peak resolution, accuracy and sensitivity. Kumar et al.also do not teach the importance of thermal correction of conductivitymeasurements, nor do they teach the importance of demineralizing polyol.

[0043] Thus, there remains an unmet need in the art for an inexpensiveboron detection and measurement system which can be made light-weight,compact and portable, and which is capable of accurately andreproducibly measuring extremely low concentrations of boron in wateron-line. This need is met, and the aforementioned drawbacks andlimitations of the prior art boron detectors are overcome, in whole orin part, with the low-level boron detection and measurement system ofthe present invention.

OBJECTS OF THE INVENTION

[0044] Accordingly, a principal object of this invention is to providemethods and apparatus for very low-level boron detection and measurementin aqueous solutions.

[0045] It is a general object of this invention to provide methods andapparatus for a boron measurement system for use in semiconductormanufacturing and other applications which require accurate, reliablemeasurement of boron at very low concentrations, e.g., 0.050 ppb B orless.

[0046] An additional object of this invention is to provide a boronmeasurement system for low-level measurements of boron in waterutilizing highly concentrated polyol solutions in very small quantities.

[0047] Another object of this invention is to provide a boronmeasurement system for low-level measurements of boron in waterutilizing accurate, small, periodic injections of high concentrationpolyol solution into a boron-containing aqueous sample to form anionized complex resulting in a substantial, measurable increase in theelectrical conductivity of the sample even at very low boron levels,e.g., 0.05 ppm as B or even less.

[0048] A further object of this invention is to provide a boronmeasurement system for low-level measurements of boron in water in whichthere is a mathematically correlated relationship between (a) thedifference in electrical conductivity of a boron-containing sample whichhas been mixed with a polyol and the electrical conductivity ofpolyol/boron-free sample, and (b) the concentration of boron in thesample over a boron concentration range of less than 0.050 ppb B toabout 1000 ppb B.

[0049] Yet additional objects of this invention are to provide alow-level boron measurement system which minimizes required amounts ofpolyol; corrects for conductivity effects of polyol/boron-free sample;and is not sensitive to variations in flow rates of boron-containingsamples or polyol solution.

[0050] Other objects and advantages of the present invention will inpart be obvious and will in part appear hereinafter. The inventionaccordingly comprises, but is not limited to, the methods and relatedapparatus, involving the several steps and the various components, andthe relation and order of one or more such steps and components withrespect to each of the others, as exemplified by the followingdescription and the accompanying drawings. Various modifications of andvariations on the method and apparatus as herein described will beapparent to those skilled in the art, and all such modifications andvariations are considered within the scope of the invention.

SUMMARY OF THE INVENTION

[0051] This invention is generally directed to methods and apparatus foraccurately measuring very low concentrations of boron in deionizedwater. It utilizes the chemical reaction of boric acid with a polyol,such as mannitol or other similar compounds, to produce an ionized acidproduct which can then be measured by a conductivity and temperaturedetector. Boric acid is only weakly ionized (pKa=9.23 at 25 C.) in purewater. The reaction of boric acid with a polyol, such as mannitol orsorbitol, however, forms a complex that is more strongly ionized(pKa=5.14 with mannitol) and causes a dramatic increase in electricalconductivity that can be measured at even very low levels of boron inwater if interfering or extraneous factors which can affect conductivityare either eliminated or corrected for.

[0052] Surprisingly, it has now been found that one can accuratelymeasure very low levels of boron in water by injecting very small plugsor aliquots of concentrated polyol into streams of boron containing andnon-boron containing water samples, and then measuring the conductivitydifference (delta conductivity) between such boron containing andnon-boron containing samples. Critically, it has been determined thatthe actual boron concentration remains mathematically correlated to suchdelta conductivity measurement over the industrially important low-levelrange of boron concentrations. With these present new methods andapparatus, one can accurately measure concentrations of boron in waterat very much lower boron levels than any prior art polyol conductometricmeasurement method. This new technique is as sensitive as the mostsensitive conventional laboratory boron measurement methods (,withoutpre-concentration), i.e., using inductively coupled plasma massspectrometry (ICP-MS) or ion chromatography. The present invention,while just as sensitive as ICP-MS and chromatographic techniques, isvastly simpler, less expensive to purchase, and can be easily operatedon-line by less skilled operators.

[0053] Another important aspect of this invention is the discovery thataccurate measurement of a very low level of boron requires boththoroughly deionized polyol and a technique to account for theconductivity of the polyol reagent after it is added to boron-containingwater. No prior art polyol-boron detection methods have ever recognizedthe importance of using thoroughly deionized polyol combined with thesteps of measuring the conductivity of polyol/boron-free sample andcorrecting the polyol/boron-containing sample conductivity measurementto account for conductivity increases attributable to polyol per se inorder accurately to determine very low-level boron concentrations inwater. When polyol (at the concentrations needed for sensitive borondetection) is added to boron-free deionized water, the conductivity ofthat solution is greater than the conductivity of such deionized water.If such additional conductivity is not accounted for, an error isintroduced in the final boron measurement. Simply passing concentratedpolyol solution through an ion exchange resin before use is notsufficient to remove such inherent added conductivity. This isapparently because the polyol itself ionizes to a small extent. At verylow levels of boron, failure to correct for this factor will lead to avery high percentage error.

[0054] In the present invention, polyol/boron-free sample conductivityis subtracted from polyol/boron-containing sample conductivity to obtaina “delta conductivity,” and this delta conductivity is mathematicallycorrelated to the true boron concentration of the boron-containingsample. The prior art does not combine deionized polyol with injectingconcentrated deionized polyol into the flowing deionized (DI) water andwith using “de-boronated” DI water to establish a base line. It is notobvious from the prior art that such combination enables precise boronanalyses at the very low levels obtained with the apparatus and methodsof the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055]FIG. 1 is a simplified process flow diagram of one preferredembodiment of the present invention.

[0056]FIG. 2 is an illustrative plot of delta conductivity data versusboron concentration over the range of about 0-10 ppb as boron.

[0057]FIG. 3 is an illustrative log-log plot of delta conductivity dataversus boron concentration.

[0058]FIG. 4 is an illustrative graph of linearized response data versusboron concentration.

[0059]FIG. 5 is an illustrative graph of concentration response versusboron concentration at very low boron concentrations of about 0-2.5 ppbas boron.

[0060]FIG. 6 is an illustrative graph of response factor plotted againstdilution factor using 0.5M mannitol as polyol.

[0061]FIG. 7 is an illustrative graph of response factor plotted againstdilution factor using 1M mannitol as polyol.

[0062]FIG. 8 is an illustrative graph for comparison purposes showingnon-optimum peak shapes obtained using a non-optimum dilution factor and0.5M mannitol (not part of this invention).

[0063]FIG. 9 is an illustrative graph showing an optimal peak shapeobtained using an optimized dilution factor and 1M mannitol.

[0064]FIG. 10 is an illustrative graph showing boron detection levelsusing 1M mannitol and a 0.32 dilution factor.

[0065]FIG. 11 is an illustrative graph showing conductivity andtemperature plots for a 0.1 ppb boron concentration.

[0066]FIG. 12 is an illustrative graph showing ion exchange columnbreak-through data, taken from prior art.

[0067]FIG. 13 is an illustrative graph comparing delta conductivity datagenerated utilizing a prior art process with delta conductivity datagenerated using the methods and apparatus of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0068] The present invention is directed to methods and apparatus foraccurately measuring very low levels of boron in ultrapure water andhaving particular utility in manufacture of semiconductors. The presentinvention also overcomes limitations of prior art processes and devicesthereby making possible rapid, inexpensive measurement of boron inultrapure water at concentrations ranging from about 0.01 to 1000 ppb asboron with equipment which is relatively compact and inexpensive, andwhich can be run on-line by relatively unskilled operators.

[0069] Large amounts of ultrapure water are required in processes tomanufacture semiconductors. Boron is a contaminate that must generallybe removed to very low concentrations. As previously discussed, boron isa silicon semiconductor p-type dopant used in manufacture of siliconsolid state electronics and functions as a principal charge carrier.Therefore, boron must not be added inadvertently during themanufacturing process. S. Malhotra et al. reported in their paper,“Correlation of Boron Breakthrough versus Resistivity and DissolvedSilica in a RO/DI System” (Ultrapure Water, 1996. 13(4): pp. 22-26) thatboron was the first ion to breakthrough ion exchange resin beds whenthey switched to thin-film-composite (TFC) reverse osmosis membranes.The introduction of TFC reverse osmosis (RO) membranes to replacecellulose acetate RO membranes was very effective in reducing the silicapassage (by a factor of 58) but less effective in reducing boron passage(by a factor of only 1.7).

[0070] The first ions to leak from mixed ion exchange resin beds whichfollow RO units are weakly ionized compounds such as dissolved silicaand boron. With TFC RO membranes, silica passage is much less than boronpassage. Boron is therefore often the first ion to breakthrough mixedion exchange resin beds in water purification systems that use TFC ROmembranes. This is especially true if the feedwater has high levels ofboron in it. Adsorption of borate ion on anion exchange resins in thehydroxide form is the most common method to remove boron from ultrapurewater. With usage, borate ion is one of the first ions to leak from theresin, rapidly reaching unacceptable levels.

[0071]FIG. 12 for example shows water resistivity, boron concentration,and silica concentration as a function of the amount of water passedthrough a mixed bed ion exchange column in a semiconductor factory asreported in the above-mentioned S. Malhotra et al. paper using TFC ROpretreatment. Typical concentrations of borate in ultrapure,semiconductor-grade water are 0.05 to 10 parts per billion as B. Theselevels can currently be measured with an ICP-MS (inductively coupledplasma and mass spectrometer) detector, but such equipment is veryexpensive, requires a skilled operator to achieve good results, andresults can not be acquired very quickly. A typical acceptable upperallowable limit of boron concentration in water for siliconsemiconductor manufacturing is from 0.1 ppb to 5 ppb. Because thebreakthrough of B to unacceptable levels can happen quickly, anautomated on-line detector is required to solve this need for frequentanalysis on a real-time basis. Prior to the present invention, theproblem of real-time, cost effective and accurate on-line processmeasurements of extremely low concentrations of boron in water had notbeen solved. The analyzer and methods of the present invention, however,are suitable for real-time, rapid, inexpensive on-line measurement ofboron in ultrapure water used for example to manufacture semiconductorelectronic circuitry.

[0072] In general, the method of the present invention comprisesinjecting a very small slug (less than 100 microliters) of aconcentrated (greater than 0.1 molar) intrinsic polyol from time to timeinto a flowing stream of deionized water which it is desired to analyzefor ortho-boric acid and/or the ionization products (anions) thereof.Such stream flows at about 100 microliters per minute. From time to timesuch stream, before intrinsic polyol injection, is diverted through aboric acid/borate selective (chelating) anion exchange resin to producea stream substantially free of boric acid respectively borate butotherwise unchanged from the boric acid/borate-bearing stream. Theelectrical resistance (or electrical conductance) of the boron-bearingDI water stream, after injection of intrinsic polyol, is measured somedistance downstream of the polyol injection point very frequently, e.g.,100 or more times per minute. The maximum value of the conductance foundis taken to represent the combined conductance of polyol-boron complexand polyol not complexed with boron. The contribution to conductancefrom polyol not complexed with boron is obtained by injecting the samevolume, same concentration of the same intrinsic polyol into the streamwhich has been substantially freed of boron, flowing at the same rateand measuring the representative conductance in the same manner asabove. The so-obtained conductance of the polyol-boron-free stream issubtracted from the conductance of the polyol-boron-bearing streamgiving a corrected conductance representing therefore only theconductance contributed by the polyol boron complex. Such correctedconductance is found to be uniquely and precisely and quantitativelyrelated to the true borate content of the DI water analyzed, down tolevels approaching 0.001 parts boron (as boron) per billion parts water,i.e., down to 0.001 micrograms boron (as boron) per liter of DI water.

[0073] As used above “intrinsic polyol” means polyol which issubstantially free of all ions other than those due to the intrinsicionization of the polyol per se. In this context, “substantially free ofall other ions” means that the measured conductivity of the polyol isthat solely of the polyol expressed to whatever significant figures arerequired for precise boron analysis at the boron concentration measured.Intrinsic polyol may generally be obtained by passing a solution of suchpolyol through a mixed bed of thoroughly regenerated strong acid cationexchanger, strong base anion exchanger and borate selective anionexchanger.

[0074] Similarly, with respect to the DI water stream being analyzed“substantially free of boric acid respectively borate” is a relativeterm meaning that after boron removal the resulting stream must have aborate/boric acid content much less than the DI water stream, e.g.,{fraction (1/10)}^(th) as much. Thus, if it is desired to determine0.001 ppb of B in the DI water stream, then the same stream relativelyfree of B should have no more than about 0.0001 ppb boron.

[0075] To obtain precise analyses of very low levels of boron, it isnecessary to correct for the intrinsic dissociation/ionization of thepolyol. In other words, such dissociation/ionization should benegligible at the boron levels to be detected and at the concentrationsof polyol used. Other polyols may be superior to mannitol in thisrespect. In accordance with the teaching of this invention, suchintrinsic dissociation/ionization of any polyol may be easily determinedby anyone of ordinary skill in the art, e.g., by passing a solution ofthe polyol through the mixed bed ion exchanger (16 of FIG. 1), injectingthe latter into DI water which has passed through column 3, andmeasuring the resulting peak conductivity.

[0076] This invention will be better understood by reference to FIGS.1-13 which are discussed in more detail below.

[0077]FIG. 1 is a schematic process flow diagram of one typicalpreferred embodiment of this invention. As seen in FIG. 1, a deionizedbut boron-containing water sample flows into boron measurement system 50through conduit 1. To generate a boron-free water sample, 3-way valve 21is initially positioned and adjusted to divert sample flow from conduit1 into conduit 2, through a boron removal, preferably boron specific,resin column 3, then into conduit 4, through 3-way valve 21 into conduit5, which includes a back pressure restriction (not shown), and finallyto waste. Column 3 preferably contains a special boron-removal materialsuch as Amberlite IRA-743T, manufactured by Rhom and Haas Company,Philadelphia, Pa., U.S.A. and (optionally but less preferably) a nixedstrong acid and strong base ion exchange resin. A portion of boron-freesample is withdrawn from conduit 5 into conduit 6. This can be doneeither by action of downstream sample pump 19 or by means of anappropriately sized restrictor (instead of pump 19) which operates inconjunction with a pressurized feed into conduit 1 to control fluid flowrate. Withdrawn boron-free sample in conduit 6 is passed through amulti-port valve device or valve system, which as shown in FIG. 1 mayadvantageously comprise a six-port injection valve 24, and then passedout to conduit 8, which may be a delay coil, and then into conductivityand temperature measuring cell 23. A boron-free sample leaving cell 23continues into conduit 18, through pump or restrictor 19, and thenpasses to waste through conduit 20. In place of six-port injection valve24, one may substitute any combination of valves to achieve the samefunctions as six-port injection valve 24, as described hereinafter.

[0078] At the same time in the other portion of system 50, a polyolsolution is recirculated through mixed strong acid and strong base ionexchange resin bed 16 to keep the polyol free of contaminating ions.Resin column 16 may optionally also include a boron removal resin suchas Amberlite IRA-743T resin manufactured by Rohm and Haas Company orother equivalent boron-absorbing material. Polyol solution is stored inreservoir 25 from where it flows into pump 13, through conduit 14, intorestrictor 26, through conduit 15, into resin column 16, and throughconduit 17 back into reservoir 25. Between pump 13 and restrictor 26along conduit 14, a portion of thoroughly deionized polyol solution iswithdrawn through conduit 9 and is pushed by fluid pressure intosix-port injection valve 24. Valve 24 is initially set in a first valveposition which causes polyol solution to be pushed through injectionloop 7, back into valve 24, and out through conduit 10 to a second 3-wayvalve 22, then through conduit 12, and back into the recirculatingpolyol stream along conduit 15. Fluid pressure is higher in conduit 14than in conduit 15 due to restrictor 26 inline between them. This firstposition of six-port injection valve 24 allows the withdrawn portion ofpolyol to circulate through injection loop 7. Injection valve 24 is thenactuated for changing the first valve setting to a second valveposition, or is otherwise reset, so as to change fluid flow through thevalve device. Flow of polyol solution now enters valve 24 throughconduit 9, passes through valve 24 and exits valve 24 directly toconduit 10, no longer passing through injection loop 7. The portion ofboron-free sample water in conduit 6 now enters valve 24 and flowsthrough injection loop 7. This pushes polyol portion that was ininjection loop 7 into conduit (or delay coil) 8 and from there intoconductivity and temperature measuring cell 23, through conduit 18,through pump or restrictor 19, and then to waste through conduit 20.Electrical conductivity of boron-free sample-polyol mixture (herein the“polyol/boron-free sample”) is measured in conductivity and temperaturemeasuring cell 23.

[0079] Injection valve 24 is then actuated, or is otherwise reset, forswitching back to the first valve position. Polyol solution now entersinjection loop 7 and pushes out the boron-free sample water portion thatwas previously flowing through it. Such polyol solution next flows intoconduit 10. A 3-way diverter valve 22 is actuated or set to divert theboron-free sample to waste. When the boron-free sample is expelled,3-way diverter valve 22 is deactuated or reset so that effectivelythoroughly deionized polyol once again circulates through the injectionloop 7 and then back to the polyol deionization recirculation loop. Thisembodiment prevents the small amount of boron-free sample from mixingwith and diluting polyol solution in the recirculation loop therebypreventing dilution of polyol over time.

[0080] To measure a boron-containing deionized water sample using boronmeasurement system 50 of FIG. 1, 3-way valve 21 in the sample inlet pathis repositioned such that sample flow bypasses boron removal column 3. Aboron-containing aqueous sample flows from sample inlet 1, throughconduit 27 and valve 21, into conduit 5 with a portion flowing intoconduit 6. Such boron-containing sample portion flows from conduit 6through six-port injection valve 24, out to conduit 8, into conductivityand temperature measuring cell 23, out through conduit 18, through pumpor restrictor 19, and then to waste through conduit 20. The polyoldeionization loop during such mode is circulating polyol throughinjection loop 7 by conduit 9, into injection valve 24, throughinjection loop 7, back to injection valve 24, through conduit 10,through 3-way diverter valve 22 to conduit 12, and then back into thedeionized polyol circulation loop. Injection valve 24 is then actuatedor reset for changing the first valve setting to a second valve positionso as to change fluid flow through the valve. The boron-containingsample water portion in conduit 6 now enters injection valve 24, goesinto injection loop 7, thereby pushing the plug of polyol solutionpreviously in injection loop 7 out of injection loop 7 and into conduit8, through conductivity and temperature measuring cell 23, throughconduit 18, through sample pump or restrictor 19, and out conduit 20 towaste. As the boron-containing sample and polyol mixture (herein the“polyol/boron-containing sample”) passes through conductivity andtemperature detector 23, the electrical conductivity and temperature ofthe polyol/boron-containing sample is measured.

[0081] Six-port injection valve 24 is then returned to the first valveposition. This causes the boron-containing sample in conduit 6 to enterinjector valve 24, pass directly through it and exit into conduit 8,through cell 23, conduit 18, pump/restrictor 19 and conduit 20 to waste.The deionized polyol solution in conduit 9 enters injector valve 24,passes through it and through injection loop 7, back into injector valve24, out of valve 24 and through conduit 10. The boron-containing samplethat was in injection loop 7 is now diverted to waste through conduit 11from valve 22. When polyol again enters diverter valve 22, this valve isdeactivated or reset so that polyol flows from conduit 10, throughdiverter valve 22, through conduit 12, and back into the recirculatingdeionized polyol loop through conduit 12. Once again, this processdesign prevents the boron-containing sample from mixing into the polyolloop thereby contaminating and diluting it.

[0082] The present invention lends itself to an even more simplifieddesign based on a slight modification of the apparatus and methoddescribed above for FIG. 1. Such simplified design eliminates divertervalve 22. Such an apparatus modification can be effected by theadditional modification of using a saturated solution of polyol (e.g.,one gram-mole, 182 grams mannitol in 1000 ml water) in the recirculationdeionization loop. For this slightly modified embodiment, excess solidpolyol is maintained within polyol storage chamber 25. During operationof boron measurement system 50, the small amounts of aqueous samples(both boron-containing and boron-free) remaining in injection loop 7after the polyol solution has been injected into the conductivity cellare added back and mixed into the polyol deionization recirculationloop. Small amounts of boron contained in the boron-containing aqueoussample portions of course, complex with the polyol, but the boron isthen removed by equilibration with special boron-removing resin as partof the ion exchange resin packed in column 16 as previously described.As long as storage chamber 25 is maintained as the coolest component inthe system (polyol recirculation, boron/ polyol complex removal, anddeionization loop), polyol will not precipitate out in small orifices ofboron measurement system 50.

[0083] As previously discussed, a key aspect of the present invention isthe method to account for the conductivity of the polyol/boron-freesample water solution. No prior art practitioners of polyol-borondetection methods have taught the importance of using concentrated,thoroughly deionized polyol combined with measuring the conductivity ofpolyol/boron-free sample and subtracting such background conductivityfrom the polyol/boron-containing sample conductivity measurementaccurately to determine very low-level boron concentrations in water. Inthe present invention, polyol/boron-free sample conductivity issubtracted from the polyol/boron-containing sample conductivity, andthis conductivity differential (delta conductivity) is mathematicallycorrelated with true boron concentration of the boron-containing sample.When polyol at the higher polyol concentration range which we have foundis critically needed for sensitive detection (preferably at least 0.05 Mpolyol, more preferably at least 0.1 M polyol, most preferably at least0.3 M polyol) is added to boron-free water, conductivity of theresulting solution is significantly greater than the conductivity ofdeionized water. If this additional conductivity is not accounted for,an error is introduced in final boron measurement. Passing concentratedpolyol solution through an ion exchange resin before mixing it with asample is necessary but not sufficient to remove any additionalconductivity effect. This is apparently due to the fact that polyolitself ionizes to a small extent. At low levels of boron, withoutcorrecting for this factor the error will be very high.

[0084] For example, at 25° C. mannitol has a pKa of 13.8, and a 0.32molar solution exerts a conductivity of 0.0601 μS/cm, a deltaconductivity increase of 0.0051 μS/cm over that of deionized water(0.0550 μS/cm). In measuring very low concentrations of boron in ultrapure water, polyol/boron-free background conductivity represents asubstantial fraction of a measured conductivity signal for apolyol/boron-containing sample. When using 0.32M mannitol for measuringboron in water at a concentration level of 5 ppb, boron-freemannitol-water solution conductivity thus accounts for about eightpercent of the conductivity signal for such mannitol/boron-containingsample. At 0.5 ppb B, background conductivity of a mannitol/boron-freesample represents over half of the conductivity signal for amannitol/boron-containing sample. At 0.05 ppb B, conductivity backgroundof a mannitol/boron-free sample accounts for about 93% of theconductivity signal for a mannitol/boron-containing sample.

[0085] Accurate conductivity measurements for a mixture of polyol andboron-free water must therefore be subtracted from conductivitymeasurements of a mixture of polyol and boron-containing water todetermine accurate low levels of boron in ultrapure water. Moreprecisely, the conductivity difference (delta conductivity) between apolyol/boron-containing sample and a polyol/boron-free sample has beenfound to be mathematically correlated to the true boron concentration insuch boron-containing sample. An important aspect of this invention isthe discovery that there exists such a mathematical correlation betweentrue boron concentration and delta conductivity over the range of boronconcentrations here of interest, namely from about 0.01 to 1000 ppb asboron, more particularly over the range of about 0.01 to 10 ppb asboron, as illustrated in FIGS. 2 and 3.

[0086]FIG. 2 shows the result of measuring boron from 0.1 ppb B to 10ppb B. The vertical axis shows the delta conductivity signal afterpolyol/boron-free sample conductivity is subtracted from theconductivity of polyol/boron-containing samples, the delta conductivitybeing reported in nano-Siemens per cm. This response is slightly curved,which is explained by a chemometric analysis of the reactions in termsof conductivity. The equations describing the chemistry underlying thisanalysis are discussed below. A log-log plot of the same data as shownin FIG. 2 is presented in FIG. 3. In FIG. 3, the slope of the line is1.1373, the intercept is 1.1264, and it is substantially straight asdetermined by the Coefficient-of-Determination (R²) value of 0.9991,illustrating that the delta conductivity data can be substantiallylinearized mathematically.

[0087] A transform equation can be derived empirically or analyticallyfrom the reaction equations. The empirically determined transform isshown in FIG. 4 and is based on Log(Delta Cond.)=1.1373 Log(BoronConc.)+1.1264 or Boron Conc.=0.10223 (Delta Cond.)^((0.8792)). Thistransform of the experimental delta conductivity data values produces astraight line of measured B concentration vs. true B concentration witha slope of 1 and an intercept of zero, as seen in FIG. 4. The sametransform of the same delta conductivity data from the same boronanalyzer is shown in FIG. 5 for four lower boron standardconcentrations. FIG. 5 shows very good linearity and accuracy even atvery low concentrations of boron.

[0088] Another novel feature of the present invention is the apparatusand method for providing a reference stream of water with essentiallyzero level of boron. Such boron-free sample is required to be introducedfrom time to time into the boron detector system of this invention formeasurement of background conductivity of thoroughly deionizedpolyol/boron-free water in order to achieve accurate low level resultsin accordance with this invention. Water having zero level of boron canbe generated by the use of special boron removing resins havingboron-attracting functional groups. Rohm and Haas Company manufacturesone such resin, Amberlite IRA-743T. This resin has a polyol (glucamine)functional group attached to a cross-linked polystyrene support matrix.The improved boron removal efficacy of this special resin over aconventional strong base resin is reported in “Application of BoronSelective Resin in an Ultrapure Water System”, M. Tanabe and S. Kaneko,published in the proceedings of the 1996 Semiconductor Pure Water andChemicals Conference at pages 98-107 and in U.S. Pat. No. 5,833,846,which is incorporated herein by reference.

[0089] Still another important aspect of the present invention is theease with which it can be adapted for on-line use. The design ofapparatus which adds effectively deionized polyol to a sample must havespecific characteristics to achieve accurate low level boronmeasurements and to allow characteristics suitable for on-line processanalysis. Practical, inexpensive and low maintenance on-line processanalyzers benefit greatly from simple design, reliable operation, simpleservice and maintenance requirements, low reagent consumption, smallsample requirements and absence of need for skilled operators. In thepresent invention, changes in sample conductivity when measured at lowboron concentrations are of the order of 0.5% of the conductivity ofdeionized water. Accurately to measure such low conductivity changesrequires highly reproducible additions of essentially deionized polyol.One embodiment which is well suited for this invention is to use asix-port valve as part of a reagent loop injection device, as describedabove in connection with FIG. 1. Careful design of the reagent loop anda downstream delay coil geometry in accordance with this invention areused to control intermixing of polyol with a water sample for maximumreproducibility of results and signal-to-noise response. The injectionof a polyol plug into a water sample has been found to create mixingconditions for polyol and sample which automatically achieve maximumreproducibility of results and also make this measurement methodsubstantially insensitive to sample flow rate. This effect is due tolaminar flow profiles, surface tension, and diffusion effects whichresult in mixing that is substantially not a function of reagent andsample flow rates.

[0090] Another advantage of such loop injection technique is that sampleflow rate is not a sensitive parameter, Table 1 below illustrates thevery low sensitivity of boron response (based on delta conductivitypeaks), using the injection technique of this invention, to changes insample flow rate. TABLE 1 20 ppb Boron Injection with 0.097 μS/cm (10.3Megaohm cm) Boron Free Water Blank Sample Flow Rate Delta Peak Height(μl/min.) (Conductivity - μS/cm) 225 0.31 450 0.30 900 0.28

[0091] It is known in the art (e.g., Shaefer et al. Z analyt. Chem. 121(1941) p. 170) that iron and aluminum interfere with the determinationof B(OH)₃ by titration of cis-1,2-diol adducts. Boron, iron and aluminumare all electron deficient atoms useful as catalysts in electrophilicreactions. Iron and aluminum may also interfere with the determinationof boron according to the method of this invention. It is possible thatthe following elements may interfere with such determination:

[0092] A. Possibly very interfering: Al⁺³, Ga⁺², Zr⁺⁴, Hf⁺⁴, Sb⁺⁵, Nb⁺⁵,Ta⁺⁵, Mo⁺⁶, Mo⁺⁵ (Ga⁺³ and/or Ga⁺² could be present in interferingamounts in recycled ultrapure water from a GaAs semi-conductorfacility).

[0093] B. Possibly moderately interfering: In⁺³, Sb⁺⁵, W⁺⁶, Re⁺⁵, Fe⁺³,Zn⁺², complexes of Al⁺³, Ga⁺³ or Sb⁺⁵ with organic nitrocompounds.

[0094] C. Possibly competing with B(OH)₃ on an equal basis: Sn⁺⁴, Ti⁺⁴,Re⁺³, Fe⁺², Pt⁺⁴.

[0095] If the method and apparatus of the present invention give resultsat very low levels of boron in any given water which are in conflictwith other methods and apparatus (e.g., Inductively Coupled PlasmaAtomic Emission Spectrometry, or Inductively Coupled Plasma MassSpectrometry), then it may be worthwhile to suspect the presence of oneor more of the above electron-deficient cations and search for such.

[0096] Alternatively, in another embodiment of the present invention,non-ionized boric acid (i.e., in an essentially neutral solution) isallowed to diffuse across a membrane into a large excess of effectivepolyol. Such membrane should be substantially permeable to non-ionizedboric acid and substantially impermeable to polyol, to ions and tocolloids. Examples of such membranes include, without limitation,Reverse Osmosis and Nanofiltration Membranes and Perfluoro Sulfonic AcidMembranes. The driving force for diffusion of non-ionized boric acid isan activity gradient of boric acid per se. If the latter is highlycomplexed on the polyol side of such membrane, then “free” boric acid onsuch side can be very low. An increase in conductivity on such polyolside is representative of the (much lower total) concentration of boricacid on the “pure” water side of such membrane. Such polyol shoulditself contribute only a trivial (but known) amount of conductivity.

[0097] If a micro-conductivity-temperature measuring cell is used, thenthe membrane-diffusion cell can also be micro, e.g., the thickness ofeach chamber (i.e., the polyol chamber and the sample water chamber) caneach be 1 millimeter or less to facilitate diffusion within thechambers. The chambers may contain structure to support the membraneand/or to promote mixing of the contents of either or both chambers. Themembrane diffusion cell may comprise hollow fiber, hollow fine fiber or“spaghetti” membranes or a bundle of such microtubes arranged within oneor more larger tubes. The polyol solution may be in the lumen of suchmicrotube or tubes and the sample solution external to the latter.Alternatively, the sample solution may flow through such lumen in whichcase the polyol solution is external to the microtube or tubes.Preferably the polyol chamber is operated on a pulsed (stop-flow) basisand the boric acid side on a continuous flow basis. The polyol will thenapproach (but theoretically never reach) equilibrium with the inletconcentration of boric acid on the sample water side of the membrane.Periodically (after the polyol has reached a reproducible fraction ofapproach to equilibrium, e.g., 95% or 98%), the boron-laden polyol canbe pulsed through such rnicro-conductivity temperature cell which can beread as described elsewhere herein.

[0098] The boron-laden polyol can be recycled/reused by passing it,after conductivity-temperature measurement, through a mixed bed ofstrong acid cation exchange resin and strong base anion exchange resinand a bed of boron specific ion exchange resin to produce essentiallyintrinsic polyol. Alternatively such mixed bed and such boron specificresin may be mixed together.

[0099] If such polyol is inexpensive, biodegradable and non-toxic, itmay be sent to waste after conductivity-temperature measurement. Anadvantage of such membrane-diffusion system however is that polyol maybe recycled/reused, permitting use of expensive but very effectivepolyols, e.g., cyclohexanehexone hydrate.

[0100] It has been pointed out herein that it is essential to determinethe conductivity of boron-free polyol solution in order to makesensitive determinations of low level boron. Such is readilyaccomplished with the above-described membrane-diffusion cell.Conductivity and temperature of intrinsic polyol solution may bemeasured before or as such solution enters such membrane-diffusion cellor preferably by the same conductivity-temperature micro-cell used tomeasure boron-laden polyol. In the latter case, when boron-laden polyolis forced out of the membrane-diffusion cell by intrinsic polyolsolution, the highest conductivity measured corresponds to boron-ladenpolyol and the lowest conductivity to intrinsic polyol solution.

[0101] Alternatively, instead of pulsed polyol solution in one chamberof the membrane-diffusion micro-cell, the polyol chamber may be operatedcontinuously, countercurrent to the boron containing water stream. Thepolyol solution may flow at a slower rate, even a much slower rate, thanthe opposing water stream. Whether pulsed or continuous, the twochambers of the membrane-diffusion cell may be linear or structured,e.g., in spiral or tortuous path form.

[0102] The apparatus of FIG. 1 may be readily modified to include such amembrane-diffusion micro-cell. Boron-removing resin bed 3 and three-wayvalve 21 may, if desired, be retained as an additional check on themeaning and/or accuracy of any boron determination. Boron removing bed3, generally containing only boron specific resin, removes only boronand little, if any, other electrolytes including ions which mayinterfere with a polyol-boric acid determination. As pointed out above,a membrane-diffusion cell should eliminate any such interference as wellas having other advantages.

[0103] In addition to diffusion of non-ionized boric acid through themembrane of the above-described membrane-diffusion micro-cell, waterwill also diffuse through the membrane in response to any osmoticpressure difference generated by polyol solution. For example, a 1 molarsolution of mannitol or sorbitol has an osmotic pressure of about 25atmospheres. If the pulsing of the polyol solution is carried out on acontrolled time basis, then the amount of water diffused and anydilution of the polyol will be reproducible. Alternatively, in suchstopped flow mode, flow may be stopped by valves at each end of thepolyol chamber of the membrane-diffusion cell (e.g., by solenoid valves)in which case the trans-membrane pressure will rapidly build up to theosmotic pressure, and water flow (but not boric acid flow) will ceaseacross the membrane.

[0104] As discussed herein, the purpose of boron removing resin 3 inFIG. 1 hereof is typically to remove solely boron, but no otherelectrolytes, to provide a boron-free water sample to be used forcorrection of the conductivity of a boron-bearing water sample. On theother hand, the latter may have a concentration of non-boronelectrolytes sufficient to interfere with the accurate determination ofvery low levels of boron and/or, as also discussed herein, may containinterfering ions. In either case, it is preferred (according to anotherembodiment of this invention) to remove at least partially allelectrolytes except boric acid and its anions at sample inlet 1 ofFIG. 1. It is now disclosed, according to this embodiment, that aboron-bearing but substantially interference-free sample stream can beprepared by passing such sample stream through a mixed bed resinconsisting of a strong acid cation exchanger and an anion exchangerwhich does not absorb boric acid but does absorb free acids strongerthan boric acid. Amberlite IRA68 (Rohm and Haas Company, Philadelphia,Pa., U.S.A.) based on N-(acrylamidopropyl)-N,N-dimethyl amine is anexample of such an exchanger. It is said to be the most basic of theso-called weakly basic anion exchange resins, sufficiently basic to forma salt with CO₂ (carbonic acid) but not with SiO₂ (silicic acid) andtherefore also not with boric acid. One of ordinary skill in the art canreadily test other anion exchange resins by means well known in the artfor many years to determine if they have the property of notsubstantially absorbing dilute boric acid but substantially absorbingacids stronger than boric acid. Amberlite IRA68 is a gel-type anionexchanger. There may be an advantage to using a macroporous (sometimescalled macro-reticular) equivalent of IRA68 to improve the removal oftrace interfering colloids. Similarly it may be advantageous to use amacroporous strong acid cation exchanger in such pretreatment bed.

[0105] Following such pretreatment according to this aspect of theinvention, the effluent of such pretreatment is preferable divided intotwo fractions, one passing directly to the polyol-boron measuring unitper se and the second fraction through boron removing means 3 of FIG. 1.

[0106] The cation exchange resin of the above described pretreatmentsystem (for removing electrolytes stronger than boric acid), may alsoinclude cation chelating cation exchange resins, cation exchange resinscontaining phosphoric or phosphinic acid groups or even carboxylic acidgroups.

[0107] The apparatus and method of this invention may advantageouslyincorporate a device and method alternatively to supply a standard,calibration and/or test sample of boric acid of known, very lowconcentration. Suitable devices and methods are described in U.S. Pat.Nos. 5,837,203 and 5,976,468, which by reference thereto areincorporated in their entireties into the present invention. In essencesuch applications pertain to analysis apparatus for analyzing fluidflowing between an inlet and a drain, the apparatus comprising:

[0108] (a) an assembly adjacent to a fluid region defined in part bysuch assembly, the assembly including an input port fluidically coupledto the fluid inlet, to the fluid region and to an output port, theoutput port defined by a first conduit having a hollow interior, aninlet end in the fluid region and an outlet end external to the fluidregion and

[0109] (b) an analyzer linked in-line with the fluid stream, theanalyzer including an analyzer inlet fluidically coupled to the outletend of the first conduit, whereby an analysis fluid flow path is definedfrom the fluid inlet seriatim through such inlet port, such fluid regionand such inlet end of such first conduit, such hollow interior of thelatter, such outlet end of such first conduit, such analyzer inlet andsuch analyzer.

[0110] The above mentioned assembly and related components must ofcourse be made of materials which do not sorb boric acid or its anions,contribute boric acid or its anions or otherwise change the electrolyteconcentration of the water being analyzed for boron. Polyvinylidenefluoride and polypropylene appear to be suitable materials. Glassappears not to be suitable.

[0111] By comparison, prior art continuous blending or mixing methodsfor conductometric polyol based boron detectors are relatively sensitiveto variations in flow rates of either polyol reactant orboron-containing water sample stream. Standard flowmeters and meteringvalves are typically used to proportion the two flows. Because it isdifficult and expensive to control very small flow rates of reagent andsample, such other methods require large amounts of polyol reagent.Compared to the polyol injection method of the present invention, theamounts of polyol needed for blended stream flows are greater andnegatively affect the viability of on-line boron measurement. Bycontrast, the reagent loop injection method incorporated into thisinvention, as previously described, requires only very small volumes ofpolyol reagent, typically on the order of less than 25-50 microlitersper injection. Such small polyol volumes make it possible, indeednecessary, to use a correspondingly small injector valve, sample tubinghaving a small inside diameter, small mixing tee, small delay coil, anda micro-conductivity and temperature sensor to achieve good detectionsensitivity.

[0112] The conductivity cell used in the present invention must be verysmall to achieve conductivity peak resolution sufficient enoughaccurately to detect a peak response and to minimize reagentconsumption. The volume of the conductivity cell sample chamber used ingenerating data for the present invention is for example 2 microliters,and the inside diameter of the conductivity cell flow path is 0.5 mm.The cell constant is 1.0 cm⁻¹.

[0113] The electronics used in this invention must very accuratelydetermine small changes in conductivity. For example, to achieve a lowerlimit of detection of 0.05 ppm boron (based on three times the standarddeviation of measurement), the electronics and the conductivity sensormust have a minimum standard deviation of about ±0.03 nS/cm. This levelof precision can be achieved for example by using bipolar pulseconductivity measurement methods. A reference describing this type ofconductivity measurement is Geiger, R. F., Microcomputer-ControlledInstrumentation for Analytical Conductance Measurements using theBipolar-Pulse Technique (Doctoral Thesis University of Illinois atUrbana-Champaign, 1983), which publication is incorporated herein byreference.

[0114] As discussed above, construction of a commercially practicalon-line boron analyzer which is sensitive enough to be of practical useto monitor ultrapure water for use in the semiconductor industryrequires a low-volume conductivity cell, very sensitive and very stableelectronics, a micro-volume polyol injection device, and a method tocorrect conductivity data as a function of temperature. Such a boronanalyzer in accordance with this invention is small and designed so asnot to consume large amounts of reagent. A boron analyzer of this designuses, for example, 330 ml of polyol reagent over a three month period ofon-line operation. Such amount of reagent is easily contained as part ofthe analyzer. This feature of the invention is in sharp contrast to thelarge amounts of reagent required for prior art boron analyzers.

[0115] Another novel aspect of the present invention involvesrecirculating polyol through a deionization resin to remove residualconductivity from all other ionic sources such as carbon dioxide andbicarbonate. Tests using a mixed strong acid and strong base ionexchange resin in column 16 of FIG. 1 showed that CO₂ and bicarbonatewere effectively removed. Polyols typically have a pKa on the order of12 to 14, and are therefore are not effectively removed by mixed strongacid and base ion exchange resins. The concentration of recirculatingpolyol is preferably maintained as high as possible, the concentrationbeing limited only by solubility or viscosity limits. In a preferredembodiment, solid polyol is maintained in polyol storage tank 25 inequilibrium with recirculating and deionized polyol reagent solution.This embodiment allows removal of one 3-way valve (valve 22 in FIG. 1)in the apparatus, but increases the risk of polyol precipitation in asmall orifice, valve, or tube. Smaller injection volumes of higherconcentration polyol solutions are desirable because aqueous samplesanalyzed will be less diluted by polyol injections.

[0116] Still another novel aspect of the present invention involvesmaximizing conductivity peaks by optimizing polyol injection dilutionfactor. When a plug or aliquot of polyol reagent is injected into thedelay coil (conduit 8 in FIG. 1), dispersion of polyol occurs throughthe delay or reaction coil extending to the conductivity and temperaturemeasuring cell (reference number 23 in FIG. 1). This dispersion causesthe concentration of polyol to decrease as it mixes into the sample.Such lowered polyol concentration, in turn, causes the conductivity peakto decrease. Dilution (D) can be modeled as D=C₂/C₁, where C₁ is theconcentration of the original polyol solution and C₂ is the polyolconcentration after dispersion. The concentration of boron in aboron-containing sample will obviously also be diluted as the polyoldisperses in it. If the initial boron concentration is B_(t), then theconcentration of the diluted dispersion will be (B_(t))(1−D). If theinitial concentration of polyol is L, then the concentration of thepolyol after it has dispersed with the water sample at the conductivitycell will be (L)(D). When these equations are incorporated into themathematical model of the chemistry, the optimum dilution factor can becalculated and plotted as shown in FIGS. 6 and 7, which show an optimummixing ratio for a maximum response factor or sensitivity of the borondetector as a function of dilution factor. Such optimum dilution factoris an important feature of this invention.

[0117] Injecting NaCl solution can be used directly to measure dilutionfactor, which can then be changed by adjusting the length and theinternal diameter of the mixing coil empirically to provide the bestmixing ratio for the particular concentration of the polyol. FIGS. 8 and9 illustrate the effect dilution factor has on conductivity peak shapeas a function of time. The poorly-defined peak shapes of FIG. 8 cause adecrease in precision, while the well-defined peak shape in FIG. 9improves the precision of the boron analyzer.

[0118] Yet another novel aspect of the present invention involvesmaximizing conductivity increase by optimizing the concentration ofpolyol in polyol/boron-containing solutions. We have found that there isan optimum concentration for each polyol compound that gives a maximumconductivity increase when a complex is formed with boron. Additionallywe have found that lower optimum mixing ratios lead to lowerconductivity of the boron-free sample polyol solution referencemeasurement, thereby increasing accuracy and improving the limit ofdetection. Table 2 illustrates these relationships. TABLE 2 PolyolConcentration and Optimum Mix Ratio Polyol-Boron Free WaterConcentration Optimum Conductivity Increase, Polyol (Molarity) Mix RatioCompared to DI Water at 25° C. Mannito 1 M 0.3 5.1 nano-Siemens Sorbitol2 M 0.1 3.5 nano-Siemens

[0119] The ability of two very weakly acidic molecules, specificallyboric acid and certain polyhydroxy (“polyol”) compounds, to react andform a more highly dissociated acidic species, as illustrated below, isthe basis for sensitive (preferably temperature corrected)conductometric detection of boron in deionized water according to thisinvention.

[0120] It is convenient to regard the reaction as one between metaborateanion, BO₂ ⁻ and a diol as shown by the following equation in which itwill be understood that at each junction of three lines there is acarbon (C) atom:

[0121] or symbolically

3LH₂O+2B⁻⇄LB⁻H₂O+L₂B⁻+2H₂O

[0122] where

[0123] represents a polyol symbolically C₂(OH)₂ or LH₂O where “L”represents the moiety “C₂O”, i.e.,

[0124] and “B” in the symbolic equation represents the metaborate anionBO₂ ⁻.

[0125] Polyol molecules with a cis arrangement of hydroxyl groupsusually provide the primary requisite to produce a large change inconductivity upon reaction with boric acid and/or its anions. Polyolseffective in the present invention satisfy these requirements includingbut not limited to mannitol, sorbitol, xylitol, arabitol,alpha-mannitan, N-ethyl-meso tartarimide,cis-2-methyl-2,3-dihydroxytetramethylene sulfone,cis-1,4-dimethyl-2,3-dihydroxytetramethylene sulfone, hydratedtriquinoyl (cyclohexanehexone hydrate), catechol, 3-nitrocatechol,pyrogallol and hexahydroxy benzene. Many other suitable polyols are wellknown in the art. See, for example, Steinberg ed. “Organo-boronChemistry,” pages 661 to 675. Of the polyols listed in Steinberg, thosehaving a Δ (delta) of at least about 100 at a concentration of about 0.5molar are preferred, those having a Δ of at least about 300 at suchconcentration are more preferred, and those have a Δ of at least about600 at such concentration are most preferred, where Δ is defined inSteinberg as: conductivity of polyol-boric acid solution minus the sumof the conductivities of the individual polyol and boric acid solutionsin Kohlrausch-Holburn units×10⁶.

[0126] For use with the boron measurement system of this invention, itis preferable to select a polyol which has a very low inherent(intrinsic) electrical conductivity in solution and a high associationconstant for boric acid and/or its anions. Such property lowersbackground electrical conductivity and improves the low end detectionlimit. It is also preferable to use a polyol which has a high solubilityin water, preferably at least 0.1 gram moles/liter, more preferably atleast 0.3 gram moles/liter, most preferably at least 0.5 grammoles/liter. Especially preferred are polyols having a solubility of 1gram mole/liter or more. As the concentration of polyol increases,response factor and low limit of detection improve when, as ispreferred, the polyol is intrinsic, that is substantially free of anyions other than those produced by the intrinsic (inherent) dissociationof the polyol per se. Such extraneous (foreign) ions are preferablypresent at concentrations of about one-tenth or less than ions producedby such intrinsic dissociation of polyol.

[0127] The polyol should also be inexpensive, non-toxic and stable.

[0128] The chemical reactions which underlie the boron measurementsystem of this invention can be modeled to predict, at leastqualitatively, conductivity response from addition of polyol toboron-containing water as functions of polyol and boron concentrations.An inverse equation can be used to calculate/estimate boronconcentration based on conductivity increase of polyol/boron containingwater compared to a polyol-boron-free sample of such water.

[0129] The fundamental equations may be written:

HBO₂⇄BO₂ ⁻+H⁺ or symbolically BH⇄B⁻+H⁺  1.

[0130] For the above, it has been found convenient to represent boricacid as metaboric acid HBO₂ and its anion as the metaborate anion BO₂ ⁻.Alternative representations include:

H₃BO₃⇄H₂BO₃ ⁻+H⁺  (b)

B(OH)₃+H₂O⇄B(OH)₄ ⁻+H⁺  (c)

B(OH)₃+2H₂O⇄B(OH)₄ ⁻+H₃O⁺  (d)

[0131] etc. One of ordinary skill in the art can choose whichever of theabove (or some permutation) is most comfortable.

[0132] or symbolically

C₂(OH)₂⇄C₂O₂H⁻+H⁺ or preferably

LH₂O⇄LOH⁻+H⁺ where “L” is a symbol for “C₂O”.

[0133]

[0134] or symbolically

B⁻+LH₂O→BLH₂O⁻

[0135] The above equation illustrates the convenience of assuming thatthe species which reacts with polyol is metaborate anion BO₂ ⁻. One ofordinary skill in the art can, if he or she prefers, write equation 3assuming H₂BO₃ ⁻ or B(OH)₄ ⁻ instead of BO₂ ⁻, in the former case adding1 H₂O as an additional product of the reaction and in the latter caseadding 2 H₂O.

[0136] Equation 3, written with borate anions, produces no additionalH⁺, the overall reaction scheme assuming that only Equation 1 above (orone of its equivalents) produces H⁺, Equation 1 being driven to theright by the addition of polyol, removing borate according to Equation3. If preferred, one can condense Equations 1 and 3 together writing

[0137] or any of its equivalents or symbolic equations.

[0138] or symbolically:

B⁻+2LH₂O⇄BL₂ ⁻+2H₂O

[0139] Note, based on the assumption made in writing Equation 1 above,Equation 4 also does not produce H⁺ and instead serves also to driveEquation 1 to the right by removing BO₂ ⁻.

H₂O⇄H⁺+OH⁻  5.

[0140] or if one prefers:

2H₂O⇄H₃O⁺+OH⁻

[0141] In the above symbolic equations:

[0142] BH=neutral metaboric acid, HBO₂

[0143] B⁻=metaborate anion, BO₂ ⁻

[0144] LH₂O=polyol,

[0145] LOH⁻=polyol anion,

[0146] BLH₂O⁻=boron polyol complex,

[0147] BL₂ ⁻=boron-polyol complex,

[0148] B_(t)=total boron

[0149] H⁺=hydrogen ion=hydronium ion=H₃O⁺

[0150] OH⁻=hydroxide anion.

[0151] The reaction equilibria, mass balances, charge balances andconductivity equations used to model this system are: $\begin{matrix}{{6.{~~~~}K_{a}} = \frac{\left\lbrack B^{-} \right\rbrack \left\lbrack H^{+} \right\rbrack}{\lbrack{BH}\rbrack}} \\{{7.{~~~~}K_{L}} = \frac{\left\lbrack {LOH}^{-} \right\rbrack \left\lbrack H^{+} \right\rbrack}{\left\lbrack {{LH}_{2}O} \right\rbrack}} \\{{8.{~~~~}K_{1}} = \frac{\left\lbrack {{BLH}_{2}O} \right\rbrack}{\left\lbrack B^{-} \right\rbrack \left\lbrack {{LH}_{2}O} \right\rbrack}} \\{{9.{~~~~}K_{2}} = {\frac{\left\lbrack {BL}_{2}^{-} \right\rbrack}{{\left\lbrack B^{-} \right\rbrack \left\lbrack {{LH}_{2}O} \right\rbrack}^{2}}\frac{\left( K_{2}^{\prime} \right)}{\left( {= \left\lbrack {H_{2}O} \right\rbrack^{2}} \right)}}}\end{matrix}$

[0152] Where K₂ includes the inverse square of the activity of water asindicated in parentheses on the right hand side of Equation 9. It isestimated that in a 1 molar solution of mannitol, the activity of wateris almost that of pure water. However at higher concentrations ofpolyol, also useful in this invention, the activity of water may besubstantially reduced.

K_(W)═[H⁺][OH⁻](═K_(W)′[H₂O])   10.

[0153] or

K_(W)═[H₃O⁺][OH⁻](═K_(W)″[H₂O]²)

(B_(t))═(B⁻)+(BLH₂ ⁻)+(BL₂ ⁻)+(BH)   11.

(H⁺)═(B⁻)+(BLH₂O⁻)+(BL₂ ⁻)+(LOH⁻)+(OH⁻)   12.

[0154] In the above “[ ]” indicates activities and“( )” indicatesconcentrations. It has already been pointed out that in many (but notall) cases of interest it is probably reasonable to assume thatactivities and concentrations are substantially equal.

S={(H⁺) {circumflex over ( )}_(H+)+(OH⁻){circumflex over( )}_(OH−)+(BLH₂O⁻){circumflex over ( )}_(BLH2O−)+(BL₂ ⁻){circumflexover ( )}_(BL2−)+(B⁻){circumflex over ( )}_(B−)+(LO₂ ¹⁴⁻){circumflexover ( )}_(LO2H−)}  13.

[0155] The meaning of the third term in the above equation is: “theconcentration of BLH₂O³¹ multiplied by the molar conductance of BLH₂O⁻,that is by {circumflex over ( )} subscript BLH₂O⁻.” The other terms inthe above equation have similar meanings. In most published tables ofequivalent conductivity (“{circumflex over ( )}” in the above), thevalues are in Siemens-cm²/gram-equivalent, in which case theconcentrations “( )” must be in gram-equivalents/cm³ and the specificconductivity “S” will then be in Siemens/cm. Some such tables howeverlist molar conductivities, i.e., Siemens-cm²/gram mole. One skilled inthe art should be alert to the possibility that different units areused.

[0156] It appears that the various equivalent conductivities at infinitedilution in Equation 13 are about as follows, LH₂O=mannitol or sorbitol:{circumflex over ( )}H+ 349.8 {circumflex over ( )}OH− 198.3 {circumflexover ( )}LO2H−  24 (approx.) {circumflex over ( )}B−  70 (approx.){circumflex over ( )}BLH2O−  23 (approx.) {circumflex over ( )}BL2−  16(approx.)

[0157] all in Siemens-cm²/grain-equivalent at infinite dilution at 25°C. They must be corrected for viscosity due to polyol, which formannitol or sorbitol is approximately: Molarity Viscosity centipoise 11.88 0.9 1.76 0.8 1.65 0.7 1.55 0.6 1.46 0.5 1.37 0.4 1.29 0.3 1.22 0.21.15 0.1 1.09

[0158] In the above, K_(a), K_(L), K₁, K₂, K_(W), (equilibriumconstants) are a function of temperature, correction for which can beincorporated in a suitable overall temperature correction. Theequivalent conductances are also a function of temperature, correctionfor which can also be included in such overall temperature correction.

[0159] Methods well known in the art for many years can be readily usedby one of ordinary skill in the art to measure the necessary K's,{circumflex over ( )}'s and viscosity in any solution of any polyol.Preferably such measurements are made using such polyol at whateverconcentration is intended to be used. Alternatively, a series ofmeasurements of conductivity (preferably also of pH) will permitcalculation of the necessary constants and their variations with polyolconcentration and temperature.

[0160] The foregoing equations can be rearranged to calculate boronconcentrations in the samples as a function of conductivity response toa polyol/boron-containing sample, conductivity response to apolyol/boron-free sample, temperature, dilution factor, and polyolconcentration. Note that Equation 13 above requires that the polyol andthe water sample be free of all other ions.

[0161] One of the principal benefits of the boron measurement system ofthis invention is a dramatic improvement in the smallest concentrationof boron that can be easily detected and accurately measured. Using anabove-described preferred embodiment of this invention in conjunctionwith polyol/boron-free sample conductivity background correction andhigh concentrations of intrinsic polyol injected in very small aliquotsinto boron-containing and boron-free samples allows measurement of boronat concentrations one to three orders of magnitude lower than prior artapparatus and methods using polyol/boron conductivity detection. FIG. 10shows a typical limit of detection for a boron analyzer according tothis invention to be as low as about 50 parts per trillion (0.05 ppb) asboron. Additional optimization of the boron analyzer in accordance withthe teachings of this application further lowers the limit of detectionto as low as 1-10 parts per trillion as boron, a practical low end rangebeing about 1-50 parts per trillion B.

[0162] Accurate boron concentration measurements based on electricalconductivity require compensation for temperature. This principle iseven more critical for very low level boron measurements. The chemicalequilibria, acid dissociation constants, and the molar conductances ofeach ion are all affected by temperature. FIG. 11 shows the strongeffect of temperature variations on the raw conductivity output during a0.1 ppb boron injection into a boron containing water sample. The levelof conductivity variation from thermal changes is on the same order asthe conductivity signal induced from polyol injection and reaction withlow concentrations of boron. FIG. 11 clearly illustrates the temperaturesensitivity of polyol/boron containing samples. The same figurehighlights the need for a temperature correction procedure appropriatefor the chosen boron analysis to achieve accurate low level boronconcentration analysis.

[0163] Three methods are preferably used to control effects of changingtemperature on conductometric polyol/boron analysis measurements of thisinvention. The first method requires fixing the temperature at a precisecontrolled level. This makes the conductometric boron detectionindependent of changing or different sample and ambient airtemperatures. Such method benefits greatly from the lower thermal massof a miniaturized apparatus such as practiced in this invention. Thesecond method utilizes a mathematical model which describes the kineticsand chemical reactions as a function of temperature. Fluid dynamic andchemometric models that accurately represent the functionality of themeasurement process can predict and correct for changes in temperature.A third preferred method measures the response of a boron detector toaccurate concentration standards at various temperatures and then fitsthis empirical data to a mathematical function that inputs temperatureand raw conductivity and outputs a temperature-corrected boronconcentration. When such third method is applied to the differentialconductivity signal (conductivity of the polyol/boron-containing sampleminus conductivity of the polyol/boron-free sample), the correctionbecomes automatic due to the cancellation of the temperature sensitiveconductivity of the water and polyol mixture. We have determined thatthe boron/polyol complex conductivity is not very dependent on thesteady-state temperature actually used. In this case, little or nothermal correction is needed over the typical steady-state temperaturerange of 25° C. to 45° C.

[0164] It will be apparent to those skilled in the art that otherchanges and modifications may be made in the above-described apparatusand methods for low level boron detection and measurement withoutdeparting from the scope of the invention herein, and it is intendedthat all matter contained in the above description shall be interpretedin an illustrative and not a limiting sense.

What is claimed is:
 1. Apparatus for measurement of boron at very lowconcentrations in water or other solvent comprising: (a) a liquid inputfor providing a flowing sample of said water or other solvent; (b) apolyol solution purification system for maintaining substantiallyintrinsic polyol solution, said polyol capable of forming a complex withboric acid and/or anions of boric acid; (c) a contacting system forcontacting a portion of said intrinsic polyol solution intermittentlywith said flowing sample, said contacting system enabling reaction ofboron in said flowing sample with polyol in said portion thereby formingpolyol-boron complex containing solution; (d) a conductance measuringunit for measuring electrical conductance of said polyol-boron complexcontaining solution and for measuring electrical conductance of polyolsolution substantially free from boron; (e) a correlating system forcorrelating boron concentration in said water or other solvent with saidelectrical conductance of said polyol-boron complex containing solutionand said electrical conductance of said polyol solution substantiallyfree from boron; (f) a display/recordation unit for displaying and/orrecording said correlated boron concentration; said apparatus furthercomprising at least one liquid flow system belonging to the groupconsisting of: System A: a liquid flow system for providing a flowingsample of said water or other solvent substantially free from boron,such that said contacting system alternatively contacts a portion ofsaid intrinsic polyol solution with said flowing sample of said water orother solvent substantially free from boron thereby forming said polyolsolution substantially free from boron; and, System B: a liquid flowsystem wherein said contacting system comprises a semi-permeablemembrane juxtaposed on a first face thereof by a flow path for saidflowing sample of said water or other solvent, and on a second facethereof by a flow path for said intrinsic polyol solution, saidsemi-permeable membrane being substantially permeable to boric acidand/or anions of boric acid and substantially impermeable to saidpolyol, said membrane enabling boron to pass from said flowing sample ofwater or other solvent into said intrinsic polyol solution therebyforming said polyol-boron complex containing solution.
 2. Apparatusaccording to claim 1 in which said contacting system comprises at leastone element selected from the group consisting of: an injection loop, amultiport injection valve, a delay line.
 3. Apparatus according to claim1 in which said polyol solution purification system comprises cationexchange resin in acid form and anion exchange resin in base form. 4.Apparatus according to claim 1 in which said polyol solutionpurification system comprises boron specific exchange resin. 5.Apparatus according to claim 1 in which said polyol solutionpurification system comprises a concentration control subsystem formaintaining a predeterminable concentration of polyol.
 6. Apparatusaccording to claim 1 in which said polyol has a Δ of at least about 100at a concentration of about 0.5 molar where Δ is the conductivity ofpolyol-boric acid solution minus the sum of the conductivities ofseparate polyol and boric acid solutions in Kohlrausch-Holbornunits×10⁶.
 7. Apparatus according to claim 6 in which said polyol has aΔ of at least about 300 at a concentration of about 0.5 molar. 8.Apparatus according to claim 6 in which said polyol has a Δ of at leastabout 600 at a concentration of about 0.5 molar.
 9. Apparatus accordingto claim 1 in which said liquid input includes an electrolyte removalsystem for removing from said flowing sample of said water or othersolvent electrolytes more strongly ionized than boric acid is in neutralsolutions said electrolyte removal system substantially not removingboric acid and/or anions of boric acid.
 10. Apparatus according to claim1 in which said liquid input includes an interfering substance removalsystem for removing from said flowing sample of said water or othersolvent one or more substances which interfere with said measurement ofboron, said interfering substance removal system substantially notremoving boric acid and/or anions of boric acid.
 11. Apparatus accordingto claim 1 in which said conductance measuring unit is a bipolar pulseconductivity measuring element.
 12. Apparatus according to claim 1 inwhich said conductance measuring unit includes electrodes.
 13. Apparatusaccording to claim 1 in which said conductance measuring unit does notinclude electrodes.
 14. Apparatus according to claim 1 in which saidconductance measuring unit includes a temperature measurement elementfor measuring temperature.
 15. Apparatus according to claim 1 in whichsaid conductance measuring unit further comprises a temperature controlsystem for controlling the temperature of said unit to a predeterminedtemperature.
 16. An apparatus according to claim 1 for accuratemeasurement of boron in an aqueous solution at a very low concentrationof about 0.001 ppb to about 1000 ppb as boron, said apparatus comprisingin combination: (a) a sample injection system comprising a multi-portvalve assembly capable of receiving at least two liquid feeds from atleast two valve input conduits, of outputting liquid streams to at leasttwo valve output conduits, and of alternatively channeling one oranother of the two liquid feeds to an injection loop for circulatingsaid one or another liquid feed alternatively to one or another of thevalve output conduits; (b) a sample water delivery system foralternatively supplying a boron-containing sample water portion or aboron-free sample water portion to said sample injection system; (c) apolyol supply system for supplying a portion of a polyol solution tosaid sample injection system; (d) a conductance cell for measuring theelectrical conductance of a liquid; and (e) a conduit for carrying fluidfrom said injection loop of said sample injection system to saidconductance cell.
 17. An apparatus according to claim 16 furthercomprising a fluid flow control device downstream from said conductivitycell.
 18. An apparatus according to claim 16 wherein said sample waterdelivery system comprises a sample water inlet; conduits for channelinga sample from said sample water inlet either through a boron-removingresin, or alternatively, so as to bypass said boron-removing resin; atleast a valve for controlling whether said sample passes through orbypasses said boron-removing resin; and, a fluid conduit connecting saidsample supply system to an input conduit of said injection system. 19.An apparatus according to claim 16 wherein said polyol supply systemcomprises a reservoir of concentrated polyol solution; a conduit systemfor circulating said polyol solution through a resin bed to remove ioniccontaminants; and a liquid conduit connecting said polyol supply systemto an input conduit of said injection system.,
 20. An apparatusaccording to claim 19 further comprising a liquid conduit connecting anoutput conduit of said injection system to said polyol supply system.21. A process for measurement of boron at very low concentrations inwater or other solvent, said process comprising providing an apparatuswhich comprises: input means for providing a flowing sample of saidwater or other solvent, polyol solution purification means formaintaining substantially intrinsic polyol solution, said polyol capableof forming a complex with boric acid and/or anions of boric acid;contacting means for contacting a portion of said intrinsic polyolsolution intermittently with said flowing sample, said contacting meansenabling reaction of boron in said flowing sample with polyol in saidportion thereby forming polyol-boron complex containing solution;conductance measuring means for measuring electrical conductance of saidpolyol-boron complex containing solution and for measuring electricalconductance of polyol solution substantially free from boron;correlating means for correlating boron concentration in said water orother solvent with said electrical conductance of said polyol-boroncomplex containing solution and said electrical conductance of saidpolyol solution substantially free from boron; means for displayingand/or recording said correlated boron concentration; wherein saidapparatus also comprises at least one system belonging to the groupconsisting of: System A: a system including means for providing aflowing sample of said water or other solvent substantially free fromboron, said contacting means alternatively contacting a portion of saidintrinsic polyol solution with said flowing sample of said water orother solvent substantially free from boron thereby forming said polyolsolution substantially free from boron; and, System B: wherein saidcontacting means comprises a semi-permeable membrane having juxtaposedon a first face thereof a flow path for said flowing sample of water orother solvent and on a second face thereof a flow path for saidintrinsic polyol solution, said semi-permeable membrane beingsubstantially permeable to boric acid and/or anions of boric acid andsubstantially impermeable to said polyol, said membrane enabling boronto pass from said flowing sample of said water or other solvent intosaid intrinsic polyol solution thereby forming said polyol-boron complexcontaining solution; said process comprising the further steps of: (a)providing a flowing sample of said water or solvent by means of saidinput means; (b) maintaining substantially intrinsic polyol solution bymeans of said purification means; (c) contacting a portion of saidintrinsic polyol solution intermittently with said flowing sample bymeans of said contacting means; (d) measuring electrical conductance ofsaid polyol-boron complex containing solution and of said polyolsolution substantially free from boron by means of said conductancemeasuring means; (e) correlating boron concentration in said water orother solvent with said electrical conductance of said polyol-boroncomplex containing solution and of said polyol solution substantiallyfree from boron; and, (f) displaying and/or recording said correlatedboron concentration.
 22. A method for measuring boron at very lowconcentrations in water or other solvent, using an apparatus inaccordance with claim 1, said method comprising the steps of: (a)providing a flowing sample of said water or solvent by means of saidinput means; (b) maintaining substantially intrinsic polyol solution bymeans of said purification means; (c) contacting a portion of saidintrinsic polyol solution intermittently with said flowing sample; (d)measuring electrical conductance of said polyol-boron complex containingsolution and of said polyol solution substantially free from boron; (e)correlating boron concentration in said water or other solvent with saidconductance of said polyol-boron complex containing solution and of saidpolyol solution substantially free from boron; and, (f) displayingand/or recording said correlated boron concentration.
 23. A process ormethod according to claims 21 or 22 respectively in which saidcontacting means comprises at least one entity belonging to the groupconsisting of: an injection loop, a multiport injection valve, a delayline.
 24. A process or method according to claims 21 or 22 respectivelyin which said polyol solution purification means comprises cationexchange resin in acid form and anion exchange resin in base form.
 25. Aprocess or method according to claims 21 or 22 respectively in whichsaid polyol solution purification means comprises boron specificexchange resin.
 26. A process or method according to claims 21 or 22respectively in which said polyol solution purification means comprisesmeans for maintaining a predetermined concentration of polyol.
 27. Aprocess or method according to claims 21 or 22 respectively in whichsaid polyol has a Δ of at least about 100 at a concentration of about0.5 molar where Δ is the conductivity of polyol-boric acid solutionminus the sum of the conductivities of polyol and boric acid solutionsper se in Kohlrauth-Holborn units×10⁶.
 28. A process or method accordingto claim 27 in which said polyol has a Δ of at least about 300 at aconcentration of about 0,5 molar.
 29. A process or method according toclaim 27 in which said polyol has a Δ of at least about 600 at aconcentration of about 0.5 molar.
 30. A process or method according toclaims 21 or 22 respectively in which said input means includes meansfor removing from said flowing sample of said water or other solventelectrolytes more strongly ionized than is boric acid in neutralsolutions, said means not substantially removing boric acid and/oranions of boric acid.
 31. A process or method according to claims 21 or22 respectively in which said input means includes means for removingfrom said flowing sample of said water or other solvent one or moresubstances which interfere with said measurement of boron, said meansnot substantially removing boric acid and/or anions of boric acid.
 32. Aprocess or method according to claims 21 or 22 respectively in whichsaid conductance measuring means is a bipolar pulse conductivitymeasuring means.
 33. A process or method according to claims 21 or 22respectively in which said conductance measuring means includeselectrodes.
 34. A process or method according to claims 21 or 22respectively in which said conductance measuring means does not includeelectrodes.
 35. A process or method according to claims 21 or 22respectively in which said conductance measuring means includes meansfor measuring temperature.
 36. A process or method according to claims21 or 22 respectively in which said conductance measuring means has atemperature, said conductance measuring means including means forcontrolling said temperature to a predetermined temperature.
 37. Amethod according to claim 22 for accurate measurement of boron in anaqueous solution at a very low concentration of about 0.001 ppb to about1000 ppb as boron comprising the steps of: (a) mixing a boron-containingsample water portion with a small aliquot injection of a polyolsolution, and measuring conductance of the resultingpolyol/boron-containing sample water solution; (b) mixing a boron-freesample water portion with a small aliquot injection of a polyolsolution, and measuring conductance of the resulting polyol/boron-freesample water solution; (c) determining a conductance differential basedon the difference between the conductance of the polyol/boron-containingsample water solution and the conductance of the polyol/boron-freesample water solution; and (d) relating said conductance differential tothe concentration of boron in the boron-containing sample water portion.38. A method according to claim 37 further wherein said polyol solutionhas a concentration of at least 0.05 M polyol/l.
 39. A method accordingto claim 37 further wherein the volume of each said small aliquotinjection of a polyol solution is not more than about 50 microliters.40. A method according to claim 37 further comprising the step ofpassing said polyol through a deionization resin prior to injection intoa boron-containing or boron-free sample water portion.
 41. A methodaccording to claim 37 further comprising the step of passing a portionof a boron-containing water sample through a boron-removal resin toobtain said boron-free sample water portion.
 42. A method according toclaim 37, said method further comprising a step of adjusting a polyolinjection dilution factor together with a step of maximizing anyconductance peaks for polyol/boron-containing and polyol/boron-freesample water solutions by optimizing said polyol injection dilutionfactor.
 43. A method according to claim 37, said method furthercomprising a step of adjusting a conductance response together with astep of maximizing said conductance response of thepolyol/boron-containing sample water solution by optimizing theconcentration of polyol in said polyol solution.